Transcription factors play a major role in cellular function by inducing the transcription of specific mRNAs. Transcription factors, in turn, are controlled by distinct signalling molecules. One particular family of transcription factor consists of the Signal Transducers and Activators of Transcription (Stat) proteins. Presently, there are seven known mammalian Stat family members. The recent discovery of Drosophila and Dictyostelium discoideum Stat proteins suggest that Stat proteins have played an important role in signal transduction since the early stages of our evolution [Yan R. et al., Cell 84:421-430 (1996); Kawata et al., Cell 89:909 (1997)].
Stat proteins mediate the action of a large group of signalling molecules including the cytokines and growth factors (Darnell et al. WO 95/08629, 1995). One distinctive characteristic of the Stat proteins are their apparent lack of requirement for changes in second messenger, e.g., cAMP or Ca.sup.++, concentrations. Another characteristic is that stat proteins are activated in the cell cytoplasm by phosphorylation on a single tyrosine (Darnell et al., 1994; Schindler and Darnell, 1995). The responsible kinases are either ligand-activated transmembrane receptors with intrinsic tyrosine kinase activity, such as EGF- or PDGF-receptors, or cytokine receptors that lack intrinsic kinase activity but have associated JAK kinases, such as those for interferons and interleukins (Ihle, 1995). When Stat proteins are phosphorylated, they form homo- or heterodimeric structures in which the phosphotyrosine of one partner binds to the SRC homology domain (SH2) of the other. The newly formed dimer then translocates to the nucleus, binds to a palindromic GAS sequence, thereby activating transcription (Shuai et al., 1994; Qureshi et al., 1995; Leung et al., 1996).
Stat proteins serve in the capacity as a direct messengers between the cytokine or growth factor receptor present on the cell surface, and the cell nucleus. However, since each cytokine and growth factor produce a specific cellular effect by activating a distinct set of genes, the means in which such a limited number of Stat proteins mediate this result remains a mystery. Indeed, at least thirty different ligand-receptor complexes signal the nucleus through the seven known mammalian Stat proteins [Darnell et al., Science 277:1630-1635 (1997)].
Clearly there is a need to further study the biochemistry of Stat proteins. Unfortunately current studies are seriously hampered due to the low quantities of purified protein available. Full-length cDNAs for all mammalian Stats have been cloned. In addition, certain Stat proteins have been expressed in baculovirus-infected insect cells using a His tag at the COOH-terminal end and then purified by Ni-affinity chromatography (Xu, X., et al., note 9 (1996). However, no one has reported the production of milligram quantities of activated Stat protein, nor more importantly, a purification process amenable to scaling up for such quantitative isolations.
To perform the biochemical studies necessary to understand the mechanism of the Stat-mediated signal transduction, and to configure assays useful for the detection of compounds that modulate Stat function, there remains an unfulfilled requirement for the production of large amounts of pure protein. Furthermore, there is a need for a means of specifically phosphorylating the correct tyrosine residue on a Stat protein and then separating the resulting phosphorylated Stat protein from the unphosphorylated form in quantitative yields. In addition, there is a need to produce large quantities of stable, soluble truncated Stat proteins that retain functional activities of the corresponding native Stat protein. Finally, there is a need to develop methods of isolating these functional truncated Stat proteins.
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